A common problem in the operation of power supplies is damage caused to the power supply by overload conditions. An overload condition, such as a short circuit or other low resistance path between the power supply output terminals, may result in excessive current flow through power supply components. It is possible to prevent this damage by detecting the overload condition and taking corrective action before the power supply components are harmed by excessive current flow.
In inexpensive switching power supplies, overload conditions are typically detected by monitoring the primary current pulses in the source or emitter terminal of the main switching transistor. The monitoring is commonly performed by a comparator, which compares the peak primary current pulses with a predetermined overload threshold, and which signals an overload condition when this threshold is exceeded.
Establishing an overload threshold, however, limits the useful operating range of the power supply. Using a current mode pulse width modulator to generate the primary current pulses, as is commonly done, there is an internal limit on the pulse magnitude which is never exceeded. This internal limit is set either by design within the pulse width modulator to limit peak current, or by default by means of error amplifier saturation. The overload threshold, therefore, must be set below this absolute limit to allow the comparator to detect pulses which exceed the overload threshold during overload conditions. The operating range of the power supply must then be further restricted such that current pulses remain below the overload threshold during normal operation. Therefore, the resulting operating range is constrained within a narrower range than would be achievable without the overload detection circuitry.
Such overload detection methods are further characterized by the need for very fast components. Because overload conditions may result in extremely narrow primary current pulses, the comparator and related components must be chosen so that they are capable of reacting to these very narrow pulses.
In response to the detection of an overload condition, the power supply is typically shut down to avoid damage resulting from excessive current flow. After being shut down and ceasing to produce an output voltage, some power supplies continue to operate at a reduced level. This operation may consist of automatically restarting the power supply after a predetermined delay. However, a power source is required to operate this restart or other circuitry, and the power supply, having been shut down, is unable to provide this power. Instead, the power source is typically generated by an additional transformer on the primary bias. An example of this is found in U.S. Pat. No. 4,209,826 issued on Jun. 24, 1980 to Priegnitz.
In light of the foregoing, it is desirable to have an overload detector which does not limit the useful operating range of a switching power supply. It is further desirable that the overload detector not be required to monitor the magnitude of the primary current pulses, which may become very narrow as overload conditions are approached.
It is also desirable to have a power source available while the power supply is shut down. It is further desirable that the power source be inexpensive and be provided without the requirement of an additional transformer or excessive power dissipation.